Multipulse Three-Dimensional Alignment of Asymmetric Top Molecules

We show, by computation and experiment, that a sequence of nonresonant and impulsive laser pulses with different ellipticities can effectively align asymmetric top molecules in three dimensions under field-free conditions. By solving the Schrödinger equation for the evolution of the rotational wave packet, we show that the 3D alignment of 3,5 difluoroiodobenzene molecules improves with each successive pulse. Experimentally, a sequence of three pulses is used to demonstrate these results, which extend the multipulse schemes used for 1D alignment to full 3D control of rotational motion.

Figure 1

Time evolution of $⟨{\cos }^2{\theta }_{bX,cY,aZ}⟩$ and $⟨{\cos }^2\delta ⟩$ after a sequence of pulses. Pump pulse parameters (peak intensity $I_0$, pulse width $\tau$, ellipticity parameter ${\epsilon }_X^2$ and delays $T_0$) are listed in the table, and the pulses are shown on the delay axis. The molecular frame axes are also shown. For DFIB, ${\alpha }_{aa}=21.5{\AA }^3$, ${\alpha }_{bb}=15.3{\AA }^3$, ${\alpha }_{cc}=10.2{\AA }^3$, and $A=1.74\mathrm{GHz}$, $B=0.484\mathrm{GHz}$, and $C=0.379\mathrm{GHz}$ [35].

Figure 2

Velocity maps of ${\mathrm{I}}^{+}$ and ${\mathrm{F}}^{+}$ ions from unaligned molecules and at peak alignment after each pump pulse. The top two rows show the polarizations of the pumps and probe relative to the VMI detector (blue disc) for each column beneath. The $XYZ$ axes and the desired orientation of the molecules is also shown. Note that the pump pulses, rather than the physical laboratory, define the “laboratory frame” axes $XYZ$ for practical reasons: it is much easier to rotate the molecules (by rotating the polarizations of all the pump pulses) and keep the spectrometer fixed for measuring the different projections. Circles mark annular regions over which $⟨{\cos }^2{\theta }_{\text{2D}}⟩$ is calculated; the evolution of these is shown in Fig. 3.

Figure 3

Evolution of $⟨{\cos }^2{\theta }_{\text{2D}}⟩$ for ${\mathrm{I}}^{+}$ and ${\mathrm{F}}^{+}$ ions. The laser pulse parameters are given in Fig. 1; only the first three pulses were used.

Figure 4

3D velocity distributions of ${\mathrm{I}}^{+}$, at the peaks of 1D (left, at 3.2 ps) and 3D (right, at 4.6 ps) alignment. The distributions were normalized to the peak values, and the velocities are in arbitrary units.